In order to make steel sheets lightweight, which are used for construction materials and structural members of means of transportation, such as vehicles and trains by reducing the thickness of the steel sheets, there have been many attempts to increase the strength of conventional steel. However, in the case of increasing the strength of the conventional steel, a disadvantage, wherein the ductility thereof has been relatively decreased, was found.
Hence, a lot of research on improvements in the relationship between strength and ductility has been conducted. As a result, advanced high strength steel (AHSS), using a retained austenite phase, as well as martensite and bainite, which are low temperature microstructures, has been developed and applied.
AHSS is classified into so-called dual phase (DP) steel, transformation induced plasticity (TRIP) steel, and complex phase (CP) steel. Each type of steel has different mechanical properties, that is, tensile strength and elongation percentage, according to a type and fraction of a mother phase and a second phase. In particular, TRIP steel, containing retained austenite, has the highest balance value (TS×El) of tensile strength and elongation percentage.
CP steel, among the above-mentioned types of AHSS, has an elongation percentage lower than other types of steel, and so has limited use in simple processing operations such as roll forming, while DP steel and TRIP steel, having high ductility, are applied to cold press forming or the like.
In addition to the above-mentioned types of AHSS, twinning induced plasticity (TWIP) steel (Patent Document 1), in which microstructures of steel formed of single phase austenite can be obtained by adding large amounts of carbon (C) and manganese (Mn) to the steel, is used. TWIP steel has a balance (TS×El) of tensile strength and elongation percentage of 50,000 MPa % or more, and exhibits very good material characteristics.
However, in order to manufacture such TWIP steel, the content of Mn is required to be about 25 wt % or more when the content of C is 0.4 wt %, and the content of Mn is required to be about 20 wt % or more when the content of C is 0.6 wt %. When these conditions are not satisfied, an austenite phase, causing a twinning phenomenon in a mother phase, cannot be stably secured, and epsilon martensite (ε), having an HCP structure, and martensite, having a BCT structure (α′), both of which greatly reduce processability, are formed. Thus, a large number of austenite stabilizing elements are required to be added so that austenite can be stably present at room temperature. As such, a process of casting or rolling TWIP steel having large amounts of alloy components added thereto may be difficult, due to problems caused by the alloy components, while, economically, manufacturing costs of TWIP steel may be increased.
Accordingly, so-called third-generation steel or extra advanced high strength steel (X-AHSS) having ductility higher than that of DP steel and TRIP steel, that is, AHSS, and lower than that of TWIP steel, and having low manufacturing costs, has been developed, but there has been no great achievement to the present.
As an example, a process of quenching and partitioning (Q&P) forming of retained austenite and martensite as main microstructures is disclosed in Patent Document 2, and according to a report based on using that process (Non-Patent Document 1), a disadvantage can be seen wherein, when the content of C is low (about 0.2%), yield strength is reduced to about 400 MPa, and only an elongation percentage of a final product similar to that of conventional TRIP steel can also be obtained.
Further, a method for significantly improving yield strength by increasing an amount of an alloy of C and Mn has also been introduced, but in this case, a problem that weldability is decreased due to alloy components being added in an excessive amount may occur.    Patent Document 1: Korean Patent Laid-Open Publication No. 1994-0002370    Patent Document 2: U.S. Patent Publication No. 20060011274    Non-Patent Document 1: ISIJ International, Vol. 51, 2011, pp. 137-144